1. Field of the Invention
The present invention relates to a hetero-junction field effect transistor, and more particularly, to a high-power nitride based hetero-junction field effect transistor having enhanced high frequency characteristics.
2. Description of the Related Art
Recently, a hetero-junction field effect transistor has been made of a nitride based-compound semiconductor to satisfy requirements for high frequency and high power electrical devices. Generally, since the nitride semiconductor has a wider band gap energy, a higher thermal/chemical stability, and a higher electron saturation velocity, compared with a typical semiconductor material, it has been widely applied not only to photo devices, but also to high frequency and high power electrical devices.
The nitride based hetero-junction field effect transistor has various advantages, such as a high breakdown electric field (about ˜3×106 V/cm), high electron saturation velocity (about ˜3×107 cm/sec), high thermal/chemical stability, and the like. Furthermore, due to wide band discontinuity at interfaces of the junction, a hetero-junction structure of AlGaN/GaN implemented in the nitride based hetero-junction field effect transistor can be provided with a high concentration electron, thereby enhancing electron mobility.
FIG. 1 is a view illustrating an example of a basic structure of a conventional nitride based hetero-junction field effect transistor.
Referring to FIG. 1, the conventional nitride based hetero-junction field effect transistor (HFET) 10 comprises a sapphire substrate 11 formed with a low temperature buffer layer 12. A semi-insulated or high resistance GaN layer 13 and an AlGaN layer 17 are formed on the buffer layer 12. The AlGaN layer 17 is provided at both ends of the top surface thereof with source and drain electrodes 19a and 19c, respectively, and with a gate electrode 19b between the source and the drain electrodes 19a and 19c. 
In the HFET structure, a two-dimensional electron gas (2DEG) layer 18 is formed by virtue of a hetero-junction between the GaN layer 13 and the AlGaN layer 17 having different band gaps, respectively. Here, if a signal is input to the gate electrode 19b, a channel is formed in the 2DEG layer 18, allowing electric current to flow between the source and the drain electrodes 19a and 19c. 
The GaN layer 13 is formed of an undoped GaN layer, and preferably has a relatively high resistance for preventing current leakage to the sapphire substrate 11 and for separation between elements.
However, in order to ensure a higher resistance, the GaN layer 13 comprises relatively more defects, resulting in reduction of crystallinity. As a result, there are problems of a high carrier concentration due to the defects without doping impurities, and of difficulty to provide high carrier mobility.
In order to solve the problems, a method of forming the semi-insulated GaN layer by way of two growth stages (growth at a low temperature and at a high temperature) has been recently suggested. However, even though the crystallinity of the GaN layer is enhanced by way of a secondary high temperature growth, it is difficult to substantially enhance the crystallinity, since the GaN layer grown by way of the secondary high temperature growth is based on the GaN layer grown by way of the primary low temperature growth.
As such, according to the conventional technology, due to the crystallinity of the channel layer unavoidably formed during a process for lowering the contact resistance of the GaN layer, there is a problem in that electric characteristics of DC and RF of the HFET device are deteriorated.